The eventual goal of this research is an understanding of the regulatory mechanisms which govern the life cycle of Salmonella typhimurium temperate bacteriophage P22. The studies outlined in this application are initial steps toward this goal. The expression of P22 genes after infection and in the repressed prophage state will be monitored by assaying specific P22 mRNA by means of its ability to hybridize with its complementary DNA template. Transcription of specific genes or groups of genes on the P22 chromosome will be measured by hybridizing radioactive labeled RNA of infected or lysogenic cells with segments of P22 DNA carrying only a few P22 genes. These segments will be obtained by enzymatic cleavages of P22 DNA at specific sites by a restriction endonuclease or as part of the DNA extracted from a cell carrying a partially deleted P22 prophage. This RNA-DNA hybridization assay for transcription of specific regions of the P22 chromosome together with the P22 mutants and genetic technology already available will be used to approach the following goals: (1) Determine the temporal sequence in which P22 genes function during the lytic cycle by hybridizing mRNA made at various times after infection to defined P22 DNA segments. (2) Describe mechanisms which regulate this temporal sequence of gene function by measuring transcription of specific regions after infection with P22 regulatory mutants. (3) Describe the difference between P22 transcription during a lytic infection and in a P22 repressed prophage. A general objective of this study is to provide a model system useful in suggesting investigations of regulatory interactions operating when oncogenic viruses infect or transform eukaryotic cells.